I don't agree, but don't have the proof to back up the statement, but consider the following.

The hot spot is just that, a 'spot' while the remainder of the manifold is still technically 'cold'
The coolant passages in the later manifold run underneath the intake runners & under the carb in the 'hot spot' position, thereby heating the whole thing from the bottom which is where unvapourised fuel droplets would tend to migrate.

These water heated manifolds were introduced at a time when the smog laws were becoming more & more strict. The emissions that are measured include unburned hydrocarbons. This is unburned fuel & in order to get this number down, & fuel mileage up, it is necessary to convert ALL the fuel to a gas because it is ONLY the gas that actually burns. Fuel in a liquid state does not actually burn, so unvapourised droplets were finding their way into the exhaust.
This unburned fuel was not producing useable power & that's bad for the owner [wasted fuel] & bad for the planet.

To maximise the return in an economy engine, every micro droplet of fuel must be converted from a liquid to a gas & heat is the only real way to do it. Remember that there is a substantial temperature drop as the air & fuel pass from the atmospheric pressure of the world outside the carb & manifold, to the high vacuum world inside the manifold.
The conversion from liquid to gas absorbs a great deal of heat, particularly in a near vacuum which is what you are trying to achieve in economy cruise mode, so it's necessary to replace at least some of it with manifold heating.

Using a 'winter' thermostat with the highest setting you can find will also be good, after all, ... all of the the late model, smog compliant & amazingly thrifty new-car engines run at temperatures undreamed of not so long back. These elevated operating temperatures are a part of the overall engine management strategy for good reason.

The more fuel that you can convert to a gas, the less fuel that the carb needs to pass into the airstream in order to achieve the best air fuel ratio & in the end, the purpose of the exercise is to pass the smallest volume of fuel into the intake airstream & still have the engine running well, it's really as simple as that. This is why I have been advocating warm air induction to the carb, although there is a limit to just how hot this air shouls be. It's ALL about making combustable gas from unconbustable liquid & at the very low power settings that this engine will be running at, with the relatively low gas velocities in the intake tract, this lower density of the heated intake air will have no negative effect on engine operation whatsoever.

Looking forward to seeing how this pans out.
I'd say 'more power to ya' but that's not the purpose of this exercise is it.